1. Field of the Invention
The present invention relates to a magnetic bearing apparatus for supporting a rotating object using magnetic levitation by a magnetic force of electromagnets, and more particularly to a magnetic bearing apparatus capable of accurately detecting a displacement of the rotating object based on a change in impedance of the electromagnets.
2. Description of the Related Art
A magnetic bearing apparatus has been widely used in a rotary machine, such as a turbo molecular pump, that necessarily has a high-speed rotating object. The magnetic bearing apparatus supports the rotating object by a magnetic force without physical contact. This magnetic bearing apparatus has several advantages including low friction in rotation of the rotating object, no wear particles, no need for maintenance as a result of wear of bearings, and no contamination due to a lubricant for bearings.
In the magnetic bearing apparatus, there have recently been growing needs for lower cost, smaller installation space, and an ability of higher-rotation. Under such circumstances, a technique of sensorless magnetic bearing has been employed in the magnetic bearing apparatus. The sensorless magnetic bearing detects a displacement of the rotating object without using a displacement sensor. The techniques of detecting the displacement of the rotating object include a technique using a change in impedance of the electromagnets, instead of the displacement sensor.
The impedance of the electromagnet is mainly composed of inductance elements of the electromagnet, and a change in this inductance is used for detecting the displacement of the rotating object. The inductance of the electromagnet depends mainly on a material and a shape of a core of the electromagnet, the number of turns of a coil, and a gap between the rotating object and the electromagnet. The material and the shape of the core and the number of turns of the coil are established at a stage of designing the electromagnet. Therefore, the change in inductance of the electromagnet is caused by a change in the gap between the rotating object and the electromagnet. In other words, the displacement of the rotating object causes a change in inductance of the electromagnet, and the detection of the displacement of the rotating object is realized by obtaining the change in the inductance. A displacement signal obtained is fed back, so that the rotating object can be supported by the magnetic levitation at a predetermined position without physical contact.
However, as an exciting current of the electromagnet varies, an electromagnetic characteristic of the core is actually changed. In other words, while the rotating object is not displaced, the inductance of the electromagnet is changed as a result of the change in the exciting current. Therefore, in this displacement detection based on the change in the inductance, the exiting current of the electromagnet produces a displacement detection error.
The electromagnet applies an external force to the rotating object when supporting it. Generally, a force generated by a low-frequency exciting current causes a small displacement of the rotating object, and on the other hand a force generated by a high-frequency exciting current causes a large displacement of the rotating object. Therefore, when supplying the low-frequency exciting current to the electromagnet, the displacement of the rotating object makes a greater change in inductance than that due to the change in the exciting current. In this case, the displacement detection error due to the exiting current has a small influence. To the contrary, when supplying the high-frequency exciting current to the electromagnet, the change in the exciting current makes a greater change in inductance than that due to the displacement of the rotating object. In this case, the displacement detection error has a great influence. As a result, controlling of the electromagnet bearing tends to be unstable in the high-frequency range.
Japanese laid-open patent publications No. 2004-132537 and No. 2005-196635 disclose solutions for preventing the unstable controlling of the electromagnet bearing in the high-frequency range. FIG. 10 is a view showing an electromagnetic bearing apparatus disclosed in the patent publication No. 2004-132537. A pair of electromagnets 502 and 503 are provided so as to interpose a magnetic rotating object 501 therebetween. The rotating object 501 is levitated and supported by a magnetic force generated by the electromagnets 502 and 503 at a desired position between the electromagnets 502 and 503 with no physical contact. A PWM-type driver 504 is used to supply exciting currents to the electromagnets 502 and 503. More specifically, opposing PWM voltages are applied to the electromagnets 502 and 503.
Current detectors 511 and 512 are provided for detecting the exciting currents flowing through the electromagnets 502 and 503. The detected current signals are added in an adder 505, and the resultant signal is inputted to a detection circuit 507, where the displacement information about the rotating object 501 is obtained. However, this displacement information includes the above-mentioned displacement detection error caused by the exciting currents of the electromagnets. To eliminate the displacement detection error from the displacement information, a subtractor 506 subtracts the current signals detected by the current detectors 511 and 512. The resultant signal is multiplied by a coefficient in a calculating circuit 508. The resultant signal obtained in the calculating circuit 508 is a signal corresponding to the above-mentioned displacement detection error caused by the exciting currents of the electromagnets 502 and 503. In an adder 509, the signal, representing the displacement detection error, is added to (or subtracted from) the displacement information obtained from the detection circuit 507, whereby the displacement detection error is removed from the displacement information.
An output signal from the adder 509, which contains no displacement detection error, is fed back to a controller 510. This controller 510 outputs a command signal to the driver 504 so as to cause the driver 504 to pass the exciting currents through the electromagnets 502 and 503 for supporting the rotating object 501 at a predetermined position. In this manner, the rotating object 501 is levitated by the magnetic force without physical contact.
FIG. 11 is a view showing an electromagnetic bearing apparatus disclosed in the patent publication No. 2005-196635. A pair of electromagnets 602 and 603 are provided so as to interpose a magnetic rotating object 601 therebetween. The rotating object 601 is levitated and supported by a magnetic force generated by the electromagnets 602 and 603 at a desired position between the electromagnets 602 and 603 with no physical contact. A PWM-type driver 604 is used to supply exciting currents to the electromagnets 602 and 603. More specifically, opposing PWM voltages are applied to the electromagnets 602 and 603.
Current detectors 610 and 611 are provided so as to detect the exciting currents flowing through the electromagnets 602 and 603. The detected current signals are added in an adder 605. An output signal of the adder 605 is an AM modulated wave signal containing a displacement information of the rotating object 601. This signal also contains the displacement detection error caused by the exciting currents of the electromagnets 602 and 603, as well as the previously-described example. To eliminate the displacement detection error from the output signal of the adder 605, an input signal of the driver 604 is filtered through a filter 608, and the resultant signal is modulated by an AM modulator 612. A subtractor 606 subtracts the modulated signal from the output signal of the adder 605.
The input signal of the driver 604 is a command signal for passing the currents through the electromagnets 602 and 603. Therefore, the input signal of the driver 604 can be used as an exciting current signal of the electromagnets 602 and 603. In other words, the input signal of the driver 604 itself can be used as a signal corresponding to the displacement detection error caused by passing the exciting currents through the electromagnets 602 and 603. However, an attenuation and a phase delay occur between when the input signal of the driver 604 commands so as to pass the currents through the electromagnets 602 and 603 and when the currents actually flow through the electromagnets 602 and 603, due to a frequency characteristic (transfer characteristic) determined from the driver 604 and the electromagnets 602 and 603. Therefore, from a standpoint of the input signal of the driver 604, these attenuation and phase delay also result in an attenuation and a phase delay in the output signal from the adder 605 obtained from the exciting current information of the electromagnets 602 and 603, i.e., the displacement information signal containing the displacement detection error, as well.
Generally, the frequency characteristic determined from the driver 604 and the electromagnets 602 and 603 is in a low degree. Therefore, a simple realization is easy. Thus, a filtering characteristic of the filter 608 is set to be equal to the frequency characteristic determined from the driver 604 and the electromagnets 602 and 603, and the input signal of the driver 604 (i.e., the signal corresponding to the displacement detection error) is filtered through the filter 608 and is modulated by the AM modulator 612. With this processing, the frequency characteristic of the output signal from the adder 605 and the frequency characteristic of the output signal from the AM modulator 612, from the standpoint of the input signal of the driver 604, can be matched to each other. Then, the subtractor 606 removes the displacement detection error from the displacement information by the subtraction.
The output signal from the subtractor 606 is an AM modulated signal of the displacement information signal from which the displacement detection error has been removed. Therefore, the output signal from the subtractor 606 is demodulated by a demodulator 607, so that a displacement signal is obtained. This displacement signal is fed back and compared with a target levitation position signal. A signal, generated based on the comparison with the target levitation position signal, is inputted into a compensator 609. In this manner, the rotating object 601 is supported stably by the magnetic levitation at a predetermined position.
However, in the magnetic bearing apparatus as disclosed in the Japanese laid-open patent publication No. 2004-132537, an attenuation and a phase delay occur in the displacement information obtained from the detection circuit 507 in accordance with the frequency of the displacement information, due to the frequency characteristic (i.e., transfer characteristic) of the detection circuit 507 itself. To closely remove the displacement detection error, it is necessary to cause an attenuation and a phase delay in the displacement detection error signal by the same amount of the detection circuit 507, so that the characteristics of the displacement information and the displacement detection error signal are matched to each other before they are added (or subtracted) in the adder 509. In other words, the calculator 508 is required to behave as a filter having a frequency characteristic equal to that of the detection circuit 507. However, a degree of the frequency characteristic of the detection circuit 507 is high, and it is difficult that the calculator 508 has the same filtering characteristic as that of the detection circuit 507. It is therefore difficult to accurately remove the displacement detection error, particularly in the high-frequency range.
In the magnetic bearing apparatus as disclosed in the Japanese laid-open patent publication No. 2005-196635, a degree of the frequency characteristic of the filter 608 is low. However, it is impossible to completely match the frequency characteristic of the filter 608 to the frequency characteristic (i.e., transfer characteristic) determined from the driver 604 and the electromagnets 602 and 603. As a result, it is difficult to realize more accurate displacement detection. Further, it takes a certain time to measure the frequency characteristic (i.e., transfer characteristic) of the driver 604 and the electromagnets 602 and 603 and to match the frequency characteristic of the filter 608 to the measured frequency characteristic.